1. Field of the Invention
The present invention relates to a fingerprint input apparatus which is suitable for fingerprint collation or identification.
2. Description of the Related Art
For obtaining a fingerprint as means for identifying each person, it is preferably to use a non-ink type data input apparatus which gives less mental and physical burden to a user. Further, as a result of considering the post-processing of an image, the fingerprint input apparatus is required to obtain the image on which ridges and valleys are displayed at high contrast and provide 50 .mu.m or some reading accuracy per one pixel.
The inventors of the present invention know two types of fingerprint input apparatus, the type employing the steps of applying a laser beam to materials contained in skin secretion and sensing light reflected on the materials and the type employing the steps of applying a ray of light to one surface of a glass plate on the other surface with which a finger is in contact and sensing a variation of the reflected light (see "Fingerprint Automatic Identifying Technique" KAWAGOE Masahiro, Measurement and Control, Vol. 25 No. 8 pp. 701-706). The commonly used fingerprint input apparatus employs "a prism method" relevant to the latter type. The prism method takes the steps of applying a ray of light to an oblique surface of a prism from the inside in a manner to allow the light to be totally reflected and image-form the regular reflected light from the inside of the prism on an image pickup device provided in an image-form optical system. When inputting a fingerprint image, a finger is required to be pushed on the oblique surface so that the convex portions of the fingerprint cause the ray of light to be scattered so that the ray of light is not allowed to reach the image pickup device, since the skin is in contact with the glass at the convex portions. This principle makes it possible to obtain a high-contrast fingerprint image. (see "Automatic Identifying a fingerprint pattern", written by KAWAGOE and TOHJYO, Proceeding on Information Processing Society, Computer Vision, 18-2, 1982)
Another method has been proposed wherein an image pickup device is located at the area on which no scattered light from the recessed portions of the fingerprint is allowed to reach for the purpose of improving a contrast (see "Method for detecting fingerprint information using a prism, written by SHIMIZU et al., National Meeting of Electronics and Communication Society, 1311, 1984).
The aforementioned methods have the following disadvantages.
(1) Appearance of a trapezoidal distortion resulting from various light paths from respective points on the fingerprint to the image pickup device,
(2) Overlapping of "Noise light" resulting from a residual fingerprint of a previous user, and
(3) Inability to reduce the overall system in size, resulting from the essential requirement of an optical system for forming an image on the outside of the prism.
Further, another fingerprint input apparatus has been proposed wherein a hologram is provided for inputting a two-dimensional image representing ridge portions of a fingerprint. This is devised for overcoming the adverse effect caused by the residual fingerprint (see "Personal Collation Device Using Holographic Fingerprint Sensor", written by IGAKI et al., Proceeding on Electronic, Information and Communication Society, Pattern Recognition and Understanding 88-38, 1988).
The aforementioned fingerprint input apparatus, however, each provides means having a glass plate with which a finger is in contact. Hence, it may leave the residual fingerprint on the glass plate, which results in bringing about noise light. The overlapping of the noise light with the light to be imaged thus makes it difficult to draw the features of the fingerprint. If the prism is used for the surface with which a finger is in contact, the trapezoidal distortion takes place because various light paths are provided between respective points on the fingerprint and the image pickup device. If the finger is even a bit slipped, the trapezoidal distortion may cause the fingerprint image to be further distorted. Moreover, since each person has each moisture on his finger, some fingerprints may be represented as a dim image. The inventors of the present invention know the use of a two-dimensional image sensor for directly imaging the fingerprint surface. This method, however, requires uniform lighting on the overall fingerprint for obtaining a uniform fingerprint image. If not, the uniform fingerprint image is not allowed to be obtained, resulting in improperly collating fingerprints. Further, the use of the two-dimensional image sensor enhances the cost of the apparatus.
As another fingerprint apparatus, the inventors of the present invention know that a non-contact type fingerprint apparatus is provided for overcoming the foregoing disadvantages. The non-contact type fingerprint apparatus is constructed so that a user does not need to contact his or her finger on a glass plate or the like for inputting the fingerprint. However, the non-contact fingerprint apparatus is incapable of precisely inputting a fingerprint pattern, because the non-contact feature makes it substantially impossible to contact the finger in stable form. This apparatus, therefore, has a significant problem that the positional, directional and rotational slippage of the fingerprint surface has to be reduced.
In turn, more concrete description about the aforementioned known arts will be made.
As the first related art, a fingerprint input apparatus includes a prism whose section is formed like a right-angled isosceles triangle, a light source, and an image pickup device. In operation, someone pushes his or her finger on a bottom of the prism located in opposition to the right-angled vertex. The light source applies a ray of light to the opposite side of the finger-put bottom of the prism at the 45.degree. angle of incidence. The finger image representing finger contour is reflected on the bottom and comes to the image pickup device. The image pickup device is located at 45.degree. angle against the bottom of the prism and picks up the finger image reflected from the bottom. The finger image is converted into an electric signal in the image pickup device.
As the second related art, the fingerprint input apparatus includes a prism whose section is formed like a right-angled isosceles triangle, a light source, and an image pickup device as well. In operation, someone pushes his or her finger on a bottom of the prism located in opposition to the right-angled vertex. The light source applies a ray of light to the opposite side of the finger-put bottom of the prism in a manner to keep the angle of incidence larger than a critical angle relevant to the total internal reflection of the bottom. The critical angle is used herein to denote a minimum angle against a vertical plane of the prism bottom which brings about total internal reflection. The contour of the fingerprint causes a ray of light to be irregularly reflected on the bottom. The irregularly reflected light is picked up by the image pickup device located on the same side of the light source but at the closer position than the light source. The image pickup device converts the light into an electric signal. The portions of the bottom on which the finger does not closely contact causes the ray of light to be totally reflected and come out of the prism.
As the third related art, the fingerprint input apparatus includes a lightguide plate having a hologram attached on one surface and made of glass, for example, a laser beam source for emitting a coherent laser beam, and an image pickup device. In operation, someone pushes his or her finger on a part of one surface of the lightguide plate. The laser beam source applies the coherent laser beam to the finger-put area of the surface. The contour of the fingerprint causes the coherent laser beam to be irregularly reflected. The irregularly reflected light is propagated within the light guide plate and reaches the hologram attached on the opposite surface of the finger-put surface of the lightguide plate. The hologram picks up the irregularly reflected light from the lightguide plate and applies the light to the image pickup device. The image pickup device converts the light into an electric signal. The laser beam comes out of the lightguide plate on the portions of the surface on which the finger does not closely contact, that is, the portions corresponding to the concaves of the fingerprint. It is reflected on the concaves of the fingerprint and returned into the lightguide plate. The light is, however, got out of the opposite surface of the lightguide plate, so that it does not reach the hologram.
The fingerprint input apparatus according to the first related art has an advantage that it has simple construction. However, the apparatus has a disadvantage that the image reflected on the contour of the fingerprint has a very low S/N (Signal-to-Noise) ratio, because the angle of incidence of the light against the bottom is as small as 45.degree..
The construction of the fingerprint input apparatus according to the second related art results in that the non-contact portions on the bottom between the contour of the fingerprint and the prism causes the ray of light to be totally reflected and outgo from the bottom of the prism without being irregularly reflected. It means that no light reflected from the non-contact portions is allowed to reach the image pickup device. The second related art is capable of providing a somewhat higher S/N ratio than the first related art. It, however, cannot sufficiently improve the S/N ratio in light of the fact that a slight quantity of irregularly-reflected light is allowed to reach the image pickup device.
The construction of the fingerprint input apparatus according to the third related art results in that the non-contact portions on the surface between the contour of the fingerprint and the lightguide plate cause the incident ray of light to outgo from the lightguide plate without allowing the light to reach the hologram. Hence, the third related art is capable of providing a far more excellent S/N ratio than the foregoing related arts. However, the disadvantage the third related art entails is the high manufacturing cost, because the third related art needs a dedicated laser beam source and optical devices such as a hologram.